This invention relates to semiconductor lasers and, more particularly, to an electron beam pumped semiconductor laser screen and method of producing the electron beam pumped semiconductor laser screen.
Large displays are becoming more popular with the prevalence of computer technologies. These large displays can present substantial quantities of information. For example, large displays that are bright and that offer very clear image quality are desirable in training and simulation systems for commercial and military aircraft. In addition, large displays are useful for a number of other commercial applications.
The common large area displays known today are based upon cathode ray tube (CRT) technology and have many drawbacks. The most significant disadvantage is the limited amount of light that can be generated behind the screen, which results in the display being dim and having poor color saturation for large areas. To address this problem, electron beam pumped semiconductor laser (EBSL) displays were developed to create large, bright, full color projector displays similar to the projection displays of phosphor-based CRTs. By substituting a semiconductor laser screen for the phosphor screen, exceptional image quality over theater-sized screens is produced and can be used in applications such as flight simulators, cockpit displays, electronic cinema, teleconferencing, auditorium displays, laser microscopy, optical computing, and theme park laser displays.
Conventional laser screens for EBSL displays are constructed from large, circular wafers cut from a single crystal ingot of a II-VI alloy, such as Cadmium Sulphide (CdS), Cadmium Selenide (CdSe) or Zinc Selenide (ZnSe). Typically, these wafers are 50 mm or less in diameter and approximately 1 mm in thickness. After the wafer is cut, one surface is polished to an optically perfect finish and appropriate dielectric mirror coatings are applied. The polished and coated surface is bonded to a polished sapphire substrate with an optically transparent cement and the wafer is then thinned to about 1-10 microns by lapping the exposed surface, opposite the sapphire substrate. Next, the lapped surface is polished to optical perfection, such that the wafer has uniform thickness and a smooth surface. This surface is coated with a metallic mirror film that provides an optical reflector and an electron beam current return path. Then the processed wafer may be utilized as a laser screen for an EBSL display by attaching the laser to the body of a CRT, which is evacuated for operation.
Unfortunately, the resulting EBSL displays have several disadvantages. In this regard, the cost and yield of the displays are unattractive because the process of polishing and coating the wafer requires extensive mechanical handling of the extremely fragile II-VI wafers, which are prone to damage. Because of the thickness of the single crystal wafer, very high electron beam voltages (greater than 60 kV) are required for operation. This high electron beam voltage results in the local generation of large amounts of heat, which reduces the operating lifetime of the screen, such as to less than 1000 hours. In addition, because the screens must typically be operated at cryogenic temperatures, i.e., less than 100 K, cryogenic cooling is needed. The high electron beam voltage may also lead to the production of hazardous X-rays, which requires that the laser be extensively shielded.
Large-size screens that incorporate conventional EBSL displays also have drawbacks. Besides the fact that the size of the screen is limited by the size of the single crystal wafer that can be grown, the cost and yield of producing the large, bulk single crystals of II-VI materials may be prohibitive because they are difficult and expensive to grow and fabricate, as described above. Moreover, the resolution of the large area displays is often poor because uniformity of the wafer thickness may be difficult to obtain during polishing, which results in undesirable variations in the laser output at different positions across the screen. Accordingly, while an EBSL screen offers a number of performance advantages for large displays, conventional EBSL screens are generally prohibitively expensive due to their low yield during fabrication and the cost of the cryogenic cooling and shielding that is typically required to support the high electron beam pumped voltages and the attendant high temperature operation.
These and other shortcomings are overcome by the electron beam pumped semiconductor laser screen and the associated fabrication method of the present invention, which provide a display screen that has a relatively long operating lifetime, is less expensive to produce, and operates at lower electron voltages and near room temperature conditions. In general terms, a semiconductor laser screen is provided that includes a laser cavity, which is defined by a metallic mirror and opposed epitaxially grown output mirror layers. The semiconductor laser cavity also includes a multi quantum well active gain region and an etch stop layer which may also permit the cavity to be tailored so as to support resonance at a wavelength that matches the optical gain of the epitaxially grown multi quantum well active gain region.
The multi quantum well active gain region is typically grown epitaxially on a sacrificial substrate, such as a single crystal GaAs substrate, in order to obtain high quality, lattice matched epitaxial layers. In a preferred embodiment, the layers consist of quantum wells of GaInP and barrier layers of (AlxGa1-x)InP, such as Al0.4Ga0.6InP, so as to advantageously operate in the visible, red spectrum. Moreover, the first layer epitaxially grown on the sacrificial substrate is an etch stop layer of (Ga1-xAlx)yIn1-yP, such as GaInP, that serves two purposes: (1) to act as an etch stop during the subsequent chemical etching of the sacrificial substrate, and (2) to adjust the cavity length to the correct resonance conditions by tuning the cavity length to precisely match the optical gain profile.
The output mirror is comprised of alternating layers of a first GaxAl1-xAs composition and a second GaxAl1-xAs composition, such as alternating layers of AlAs and Ga0-5Al0.5As, that are epitaxially grown on the multi quantum well active gain region. The output mirror can also include a thin, final layer of GaAs to cap the structure and prevent degradation of the GaxAl1-xAs layers. The reflectivity of the output mirror depends on the number of layers and upon the wavelength of the light, so structures can be designed and fabricated for a wide variety of wavelengths and reflectivities.
The gain layers and the output mirror are grown using epitaxial growth techniques such as metal organic chemical vapor deposition (MOCVD), Metal Organic Vapor Phase Epitaxy (MOVPE) or molecular beam epitaxy (MBE). These epitaxial growth techniques are advantageous because they are capable of the extremely high precision in the control of layer thickness required in this structure and they provide for more uniform wafer thickness than in conventional designs and thus, better resolution of the large area displays.
The semiconductor laser screen also includes a carrier that is affixed to and supports the output mirror. Further, the semiconductor laser screen includes a metallic mirror and, in one embodiment, a hybrid metallic-dielectric mirror, on the etch stop layer opposite the output mirror. As such, a laser cavity is defined between the output mirror and the metallic mirror. The metallic mirror is thin so as to permit electrons to penetrate therethrough in order to pump the multi quantum well active gain region. The semiconductor laser screen will then emit light in the red portion of the visible spectrum.
In order to fabricate the semiconductor laser screen, the epitaxially grown gain and output mirror layers are bonded to the optically transparent carrier. The carrier commonly chosen for screens is sapphire, although other materials could also be used as long as they are optically transparent, mechanically strong, and are chemically inert during the subsequent fabrication steps. An optically transparent adhesive or wafer fusion is preferably used to bond the epitaxial structure to the carrier, which will allow the emitted laser beam to pass through with little attenuation.
The sacrificial substrate upon which the gain and output mirror layers were grown is then removed by using a combination of mechanical polishing followed by chemical etching until the etch stop layer is exposed. During the chemical etching, the etch stop layer can also be etched, and may be completely removed, to precisely control the thickness of the resulting laser cavity. The metallic mirror that defines one end of the laser cavity is then deposited on the etch stop layer.
The semiconductor laser screen may be integrated into an electron beam pumped semiconductor laser, such as a cathode ray tube (CRT), which forms the basis for a projection display. This laser also includes an electron beam source for generating an electron beam that impinges upon the semiconductor laser screen in such a manner that at least some electrons of the electron beam penetrate the metallic mirror and pump the multi quantum well active gain region to generate lasing in the red spectrum. The laser further includes an evacuated tube such that the semiconductor laser screen and electron beam source are positioned at opposite ends and a deflector is positioned between them for controllably deflecting the electron beam. As such, a semiconductor laser screen can therefore be reliably fabricated according to the present invention that emits light in the red portion of the visible spectrum. As a result of its construction, the semiconductor laser screen need not consume as much power as is required for conventional laser screens, thereby permitting operation at or near room temperature and providing a relatively long operational lifetime.